Life and death in the granuloma--- immunopathology of tuberculosis

REVIEWLife and death in the granuloma:immunopathology of tuberculosisBernadette M Saunders 1,2and Warwick J Britton 1,2During tuberculosis (TB)infection,the granuloma provides the microenvironment in which antigen-specific T cells colocate with and activate infected macrophages to inhibit the growth of Mycobacterium tuberculosis .Although the granuloma is the site for mycobacterial killing,virulent mycobacteria have developed a variety of mechanisms to resist this macrophage-mediated killing.These surviving mycobacteria become dormant,however,if host cellular immunity or the signals maintaining granuloma structure wane,or if mycobacteria resume replication,leading to reactivation of TB.This balance of life and death applies not only to the mycobacterium but also to the host macrophages that may undergo apoptosis or necrosis,leading to the characteristic caseous necrosis within the granuloma,and the potential spread of TB infection.The immunological factors controlling the development and maintenance of the granuloma will be reviewed.Immunology and Cell Biology (2007)85,103–111.doi:10.1038/sj.icb.7100027;published online 9January 2007Keywords:granuloma;tuberculosis;reactivation;cytokines;chemokinesThe formation and maintenance of granulomas are essential for the control of mycobacterial infections but,paradoxically,granulomas are also responsible for the typical immunopathology caused by these infections.There are over 70species of Mycobacteria,but Mycobacter-ium tuberculosis and Mycobacterium leprae,the causative agents of tuberculosis (TB)and leprosy,are the major human pathogens.These slow-growing mycobacteria have adapted to survival within the macrophage,and this capacity for persistence of mycobacteria in the face of a potent cellular response underlies the chronic inflammatory reaction of the host.Mycobacterial infection of dendritic cells (DCs)stimulates CD4and CD8T cells,which on recruitment to the sites of infection activate infected macrophages.M.tuberculosis ,however,blocks phago-lysosomal fusion and acidification of infected phago-somes,and also partially inhibits the activation of infected macro-phages by interferon (IFN)-g ,the major effector cytokine released by Th1-like CD4T cells.As a result,some mycobacteria persist in the infected lung,leading to chronic antigenic stimulation and T-cell accumulation around macrophages.In the face of chronic cytokine stimulation,macrophages differentiate into epithelioid cells and fuse to form typical giant cells.Within the resulting granuloma,there is a balance between mycobacterial killing and survival.The local archi-tecture results in the close apposition of lymphocytes and macro-phages,and this is necessary for the activation of macrophages to kill M.tuberculosis.But the survival of some tuberculous bacilli leads to latent TB infection (LTBI),which is contained by the granulomatous process.Following acute M.tuberculosis infection,this process is adequate to control the infection in 95%of subjects,while theremainder progress to primary TB disease.There are currently two billion humans with LTBI,and reactivation of the infection occurs in 5–7%1of these subjects without,and in up to 50%with,human immunodeficiency virus (HIV)co-infection.Although granulomas can develop during other infections such as schistosomiasis and in non-infectious conditions,including sarcoido-sis and Crohn’s disease,this brief review will focus on recent deve-lopments in our understanding of mycobacterial granulomas.In particular,it will review the regulation of the adaptive CD4T-cell response essential for the formation of granulomas,the role of tumor necrosis factor (TNF)family members and chemokines,and the processes leading to sustained latency or reactivation of infection in humans.ADAPTIVE IMMUNITY AND GRANULOMA FORMATION Cellular requirements for granuloma formationThe activation of ab T-cell receptor (TcR)expressing CD4T cells is essential for the formation of granulomas during mycobacterial infections.2Additional types of T cells contribute to the immune response during M.tuberculosis infection in mice and humans,including ab TcR expressing CD8T cells,gd TcR expressing T cells and CD1-restricted CD4/CD8double negative T cells;however,granulomas develop during experimental M.tuberculosis infection of mice deficient in these cells (Table 1).3,4For example,the initial control of infection and inflammatory response is normal in CD8T-cell deficient mice,although later in infection there is increased bacterial load and lymphocyte accumulation in the lungs.3ThisReceived 3November 2006;revised 7November 2006;accepted 8November 2006;published online 9January 20071MycobacterialResearch Programme,Centenary Institute,Newtown,New South Wales,Australia and 2Discipline of Medicine,Central Clinical School,University of Sydney,Sydney,AustraliaCorrespondence:Dr B Saunders,Mycobacterial Research Programme,Centenary Institute,Locked Bag No 6,Newtown,2042New South Wales,Australia.E-mail:b.saunders@.auImmunology and Cell Biology (2007)85,103–111&2007Australasian Society for Immunology Inc.All rights reserved 0818-9641/07$/icbcontrasts with the marked susceptibility to M.tuberculosis of b2-microglobulin À/Àmice,which lack classical and non-classical major histocompatibility complex (MHC)class I molecules and therefore both CD8-and CD1-restricted T cells.5The increased iron stores in these mice,which is due to the lack of the b 2-microglobulin-associated iron transport molecule,HFE,also contribute to the failure to control intracellular mycobacterial infection.6The CD4T cells are stimulated by M.tuberculosis -infected DCs in the lymph nodes draining the lung,7and differentiate into Th1-like CD4T cells secreting interleukin (IL)-2,IFN-g and lymphotoxin-a (LT a ).Infection of DCs with M.tuberculosis or the vaccine strain Mycobacterium bovis bacille Calmette Guerin (BCG)results in upregu-lation of MHC class II and the costimulatory molecules,CD80and CD86,in a TLR2-dependent fashion 8,9and the secretion of IL-12and the pro-inflammatory cytokines.Multiple components of myco-bacteria can stimulate TLR1/2,TLR2and TLR9in vitro (reviewed in Krutzik and Modlin 10),and mice deficient in both TLR2and TLR9fail to develop normal CD4T-cell responses and are profoundly suscep-tible to M.tuberculosis infection.11DCs,rather than macrophages,are the primary source of both IL-12and IL-23during mycobacterial infection,12and their secretion is enhanced by further activation of the infected DCs by IFN-g and CD40ligand-mediated interaction from CD4T cells.13This IL-12response is downregulated by endogenous IL-10,which is also released by mycobacteria-infected DCs.8Cytokine requirementsDissection of the genetic basis for Mendelian susceptibility to myco-bacterial infections (MSMD)in humans,with marked susceptibility to BCG and non-tuberculous mycobacteria such as Mycobacterium avium,along with studies in gene-deficient mice,have helped define the cytokine requirements for the activation and function of CD4T cells against mycobacteria.Variants in five genes have been asso-ciated with MSMD,IFN-g receptor chains 1(R1)and IFN-g R2,Stat1in the IFN g R signalling pathway,IL-12p40and IL-12receptor R b 1.14As observed in IFN-g -deficient mice,subjects with defects in the IFN-g signalling pathway are profoundly susceptible to mycobacterial infec-tions,and these present at an early age and are difficult to treat.14These subjects fail to develop granulomas in tissues infected with mycobacteria,demonstrating the absolute requirement of IFN-g signalling for the activation of macrophages and granuloma forma-tion.Mice that lack IFN-g 15also fail to develop granulomas following aerosol M.tuberculosis infection,and instead of discrete granulomas composed of activated macrophages and lymphocytes,their lungs are infiltrated by neutrophils leading to necrosis and death necrosis.16The differentiation of Th1T cells is dependent on the transcription factor T-bet,17and mice deficient in T-bet have reduced IFN-g responses andincreased susceptibility following M.tuberculosis infection.18Interest-ingly,the lungs of M.tuberculosis T-bet À/Àmice develop a striking infiltrate of eosinophilic macrophages and multinucleate giant cells with neutrophils rather than well-formed granulomas.Within the granuloma,IFN-g stimulates macrophages to kill mycobacteria through a variety of pathways,including activation of phagocyte oxidase and inducible nitric synthase,19to produce reactive oxygen and nitrogen metabolites,and the upregulation of the GTPase,LRG47,which stimulates phago-lysosomal fusion.20The development of mycobacteria-specific IFN-g secreting effector T cells is also dependent on the cytokine milieu during their activation by infected DCs.IL-12and IL-23are critical cytokines for the development of this Th1-like pattern of cellular immunity.They are heterodimeric cytokines composed of a shared IL-12p40chain with IL-12p35or IL-23p19chains for IL-12and IL-23,respectively,and their receptors also share a common IL-12R b 1chain.21Humans with mutations in either IL-12p40or IL-12R b 1are deficient in both IL-12and IL-23signalling and have reduced IFN-g T-cell responses.22These subjects were identified because of increased susceptibility to myco-bacterial MSMD and Salmonella infections,but there are significant differences to those with IFN-g signalling deficiency.The infectious syndromes are not as severe and may present later in life.Following successful therapy,they do not recur as readily,suggesting that partial immunity has developed despite IL-12/IL-23deficiency.23Moreover,these subjects do develop granulomas with aggregations of lympho-cytes and macrophages in infected organs,rather than the sheets of heavily infected macrophages present in IFN-g -deficient subjects.This has led to the suggestion that the IL-12/IL-23axis is not as important in humans 23as in mice,where IL-12p40is essential for the development of competent antimycobacterial T-cell responses.24Some IL-12R b 1-deficient individuals show evidence of an IL-12-indepen-dent pathway of IFN-g production,25and this may be sufficient for macrophage activation and containment of mycobacterial infection within granulomas in some individuals.In addition,there are func-tional differences between different IL-12R b 1alleles so that the degree of signalling impairment,subsequent IFN-g deficiency and clinical phenotype varies between individuals.26IL-23is also important in the differentiation of a separate effector T-cell population secreting pro-inflammatory cytokines of the IL-17family and TNF.These IL-17-secreting T cells are implicated in the pathogenesis of autoimmune inflammatory processes in mice,27and the major role for IL-23was suggested to be the development of these pro-inflammatory T H 17cells,whereas IL-12is the critical regulatory cytokine for immunity against intracellular pathogens.Indeed,IL-12can compensate for the absence of IL-23in the control of M.tuberculosis infection;28however,this does not exclude IL-23Table 1Contribution of T-cell subsets for normal granuloma formation following aerosol M.tuberculosis infectionKO strain Survival Granuloma formationReferenceab T cell 48days Extensive inflammatory infiltration,necrotic lesions dominated by neutrophils.4CD4120days Delayed inflammation with few lymphocytes,predominantly macrophages localized as perivascular cuffing accompanied by increased neutrophils.2CD84150days Normal granuloma development initially apart from increased lymphocytes.During chronic infection,there was increased neutrophils and necrosis.60b 2m 40days Increased inflammatory infiltrate dominated by macrophages and neutrophils with caseating necrosis.Lymphocytes were restricted to the adjacent perivascular cuff.113CD1Normal Normal granuloma development.3gd T cellNormalIncreased neutrophils within granulomas,otherwise normal.114Life and death in the granuloma BM Saunders and WJ Britton104Immunology and Cell Biologyplaying a role during mycobacterial infections.Plasmid IL-23is as effective as plasmid IL-12,an adjuvant to enhance the protective effects of DNA vaccines against pulmonary TB,29and plasmid IL-23 can complement IL-12p40deficiency in the induction of protective IFN-g T-cell responses against TB.30In addition,it is now clear that the induction of IL-17-producing T-cell responses are not dependent on IL-23in vivo,but rather they develop in the absence of the inhibitory effects of IFN-g.Thus,BCG immunization of IFN-g or IL-12p40-deficient mice leads to the emergence of mycobacteria-specific T cells secreting IL-17and TNF.30The contribution of these pro-inflammatory T cells to granuloma formation and their role in protective immunity remains to be clarified.Regulation of inflammatory responsesAlthough granulomas are essential for the containment of mycobac-teria,this intense inflammatory response in the presence of persisting mycobacteria has the propensity for tissue damage.Therefore,regula-tion of the granulomatous process is essential.This occurs in part through mechanisms for regulating the expansion of activated T cells common to other infections;however,additional processes for regulating the granulomatous response have been defined.First, IFN-g itself,while essential for granuloma formation,may also limit the uncontrolled expansion of mycobacteria-reactive T cells. In the M.avium model of pulmonary infection,IFN-gÀ/Àmice accumulate large lymphocyte infiltrates in the lungs,which undergo necrosis and form cavities consistent with unrestrained expansion of the T cells.31Second,endogenous IL-10,produced during mycobac-terial infections,can reduce IL-12production from mycobacteria-infected DCs,migration of DCs to draining lymph nodes and the expansion of IFN-g-secreting T cells.32In addition,IL-10‘de-activates’macrophages and reduces the impact of IFN-g on macrophage activation and resultant mycobacterial killing.This effect is most apparent during infection with less virulent mycobacteria such as M.avium;however,IL-10-deficient mice,infected with M.tuberculosis, show a transient increase in IFN-g-secreting T cells and decrease in bacterial burden,consistent with a partial inhibitory effect of IL-10.33 An additional characteristic of T-betÀ/Àmice during M.tuberculosis infection was the increased production of IL-10in the lungs as well as reduced IFN-g production,and this IL-10may have contributed to the ineffectual macrophage responses in these mice.18Overexpression of huIL-10in antigen-presenting cells in the lungs of M.avium-infected mice resulted in markedly increased bacterial burdens with a marked increase in macrophages in the granulomas.34Macrophage apoptosis as well as the production of TNF,IL-12p40and nitric oxide were reduced,indicating that IL-10from macrophages and DCs exerts an inhibitory effect on the control of mycobacterial infection by suppres-sing macrophage effector mechanisms.More recently,IL-27has been found to provide another regulatory network for controlling inflammatory responses to mycobacteria.IL-27,which is a heterodimeric cytokine produced by DCs and macro-phages,wasfirst characterized as an immunoregulatory cytokine that increased the production of IFN-g from naı¨ve CD4T cells.35In vitro studies demonstrated that signalling through IL-27R on CD4T cells activated Stat1and T-bet,suggesting IL-27sensitized naı¨ve CD4 T cells to the Th1polarizing effects of IL-12.36Subsequently, M.tuberculosis infection of IL-27R(WSX-1)-deficient mice revealed that IL-27played an inhibitory role in the protective immune response during thefirst3months of infection,when the IL-27RÀ/Àmice showed reduced bacterial burden in the lungs.37Late accumulation of IFN-g-secreting cells in the IL-27RÀ/Àmice caused severe lung pathology.This is consistent with the lethal CD4T-cell-mediated immunopathology that developed in IL-27À/Àmice infected with Toxoplasma gondii.38This downregulation of the adaptive CD4T-cell response to infection by IL-27occurs through a variety of mechan-isms,including the IL-27-mediated inhibition of IL-2productionby CD4T cells and the increased expression of SOCS-3.38IL-27also inhibits IL-17production,independent of the IFN-g-mediated suppression of IL-17.39Finally,T regulatory cells(Tregs)may also inhibit the CD4T-cell response and limit the granulomatous inflammatory response toM.tuberculosis.Urdahl et al.40have recently shown that Foxp3-expressing Tregs are recruited to the lung during experimental TB infection in mice along with effector T cells.Depletion of Tregs before infection resulted in increased clearance of the bacteria from the lung, indicating that the Tregs were inhibiting the T-cell-effector mechan-isms.The mechanisms of this effect of Tregs during mouse TB andtheir possible role in human TB remain to be elucidated. MICROARCHITECTURE OF THE GRANULOMAThe presence of granulomas in infected tissue is a hallmark of mycobacterial disease.This requires the coordinated recruitment of macrophages and lymphocytes across the endothelium,their migration through infected tissues and aggregation to form granulomas.This permits the close interaction of antigen-specific T cells with infected macrophages, which act to contain the bacilli and prevent further spread.The cellular and chemical factors controlling granuloma formation are considerable and incompletely understood.It is clear,however,that members of the TNF family,most notably TNF and LT a,are involvedin this process.TNF family members and granuloma developmentThe TNF superfamily currently consists of19ligands and29recep-tors.41TNF ligands are type II transmembrane proteins,that assembleas biologically active trimers.TNF receptors are type I transmembrane proteins characterized by the presence of one to six cysteine-rich domains.41TNF family members have wide-ranging involvement in immune homeostasis,activation of both macrophages and T cells,andinflammation.T o date the function of at least20family membershas been examined during host responses mycobacterial infections (Figure1).Five of these molecules(soluble TNF,membrane-boundTNF(MemTNF),LT a,TNFRI and CD40)are essential for normal granuloma formation and survival following M.tuberculosis infec-tion.42–46For example,in the absence of TNF,there was a delay in theinitial recruitment of monocytes into infected tissue.42,43Despite normal activation of antigen-specific T cells,42,43this was followedby a dysregulated inflammatory response,with the influx of neutro-phils,formation of necrotic lesions,uncontrolled bacterial growthand the rapid demise of infected animals.Interestingly,mice lackingLT a,which binds to the TNFRI molecule in addition to TNFRII and HVEM,also exhibit marked susceptibility to TB infection,with a similar dysregulated inflammation and overwhelming bacterial growth despite the production of normal levels of TNF.45This demonstrated separate,non-compensatory functions for TNF and LT a,and this has subsequently been confirmed during malaria and Listeria mono-cytogenes infections.47,48Both TNF and LT a bind TNFRI and TNFRII;however,signallingfor protective effects occurs primarily through TNFRI,as TNFRIIÀ/Àmice display only minor increased susceptibility to mycobacterial infection.49LT a also binds the HVEM receptor,but the importanceof this interaction in antibacterial immunity or inflammation has notbeen established.Interestingly,TNF-and LT a-deficient micedisplayLife and death in the granulomaBM Saunders and WJ Britton105Immunology and Cell Biologymarked differences in their response to Mycobacterium leprae infec-tion.50While loss of either cytokine augments growth of M.leprae ,leukocyte infiltration in TNF À/Àmice is more extensive with some lymphocyte infiltration,while LT a À/Àmice exhibited less inflamma-tion with minimal lymphocyte infiltration.Further,while the chemo-kine responses in infected wild type (WT)and TNF À/Àmice were similar,LT a mice demonstrated five-to 25-fold lower levels of mRNA expression for the chemokines tested.Therefore,the inflammatory response of TNF-and LT a -deficient mice are not identical and are influenced by the virulence of the mycobacterial pathogen.Soluble and membrane-bound forms of TNF and LT make different contributions to granuloma formation and protective immunity.Deletion of LT a removes both soluble and membrane-bound forms of LT,whereas deletion of LT b leaves the soluble LT a trimer intact.We have previously shown that mice lacking LT a ,but not LT b ,are more susceptible to TB infection,45indicating that it is the soluble LT a that coordinates granuloma formation.TNF also has membrane-bound and soluble forms.In this instance,both memTNF and soluble TNF are required for optimal resistance to mycobacterial infec-tion.44,51MemTNF alone was sufficient for the establishment of normal granulomatous lesions and containment of bacterial growth for three months.44Further,these mice developed a normal antigen-specific T-cell response,but despite this,increased numbers of activated T cells accumulated in the lung during the chronic phase of infection.This resulted in progressive inflammation in the lung,and the memTNF mice succumbed after day 120despite control of bacterial growth.44,51Therefore,memTNF is sufficient for the initial formation of granulomas,but soluble TNF is required for long-term optimal control of mycobacterial infection.Investigation of other members of the TNF superfamily (Figure 1)has demonstrated that TNFRII,CD40L,CD95,LIGHT and CD30are required for optimal immunity to mycobacterial infection.Mice lacking these TNF family molecules showed transient or partial susceptibility to mycobacterial infection.A common feature of these molecules is their expression during the immunopathological response to TB infection.For instance,CD40L is expressed on alveolar macro-phages of tuberculoid granulomas,and increased mRNA for CD40L is present in M.tuberculosis -infected macrophages.52,53CD40–CD40L interaction induces a cascade of events,including the production ofcytokines and chemokines.The absence of CD40L impaired granu-loma formation after BCG and M.avium infection,leading to increased necrosis and impaired control of bacterial growth;however,CD40L À/Àmice survived M.tuberculosis infection despite impaired granuloma formation.46,54–56Interestingly,CD40À/Àmice were more susceptible to M.tuberculosis infection than CD40L À/Àmice,exhibit-ing marked inflammation with increased bacterial growth.46Human studies identified variants in the CD40L gene,but none of these was associated with increased susceptibility to TB.57LIGHT,which shares common receptors with LT,is required for the optimal activation of both CD4and CD8T cells in vitro .58Never-theless,LIGHT À/Àmice infected with M.tuberculosis displayed only a transient exacerbation of bacterial load at 4weeks post infection,with no change in the granulomatous response (Saunders,unpub-lished observations,Ehlers et al .59).FAS/FASL signalling is critical for T-cell homeostasis and activation of macrophage apoptosis.Mice deficient in FAS/FASL signalling controlled acute TB infection,but displayed increased pyogenic inflammation and bacterial growth during the chronic phase of infection.60Therefore,signalling through both TNFRI and FAS is required for optimal immunity to mycobac-terial infection and for regulation of the inflammatory response.Finally,antibody neutralization of the TNF family members,CD27,CD30,IBB1and OX40L,indicated that only CD30was required to control M.avium infection.61Further,CD30À/Àmice displayed reduced T-cell activation and diminished lymphocyte cuffing around granulomas,confirming its requirement for optimal granuloma form-ation.A common feature of TNF family members,in particular TNF and LT a ,in immunity to TB infection,is their regulation of chemo-kine and chemokine receptor expression,that controls the recruitment of inflammatory cells.Chemokine requirements for granuloma formationThe differential regulation of chemokines and their receptors during M.tuberculosis infection in humans is increasingly recognized (Table 2);however,the function and contribution of individual chemokines or receptors to this response is still uncertain.The establishment of chemokine gradients is crucial for the recruitment of inflammatory cells and their aggregation to form granulomas.Multiple studies have demonstrated elevated levels of chemokines,Figure 1TNF superfamily involvement in granuloma formation and resistance to mycobacterial infection.Reported:Dark gray ligands and receptors areessential for normal granuloma formation and sustained resistance to mycobacterial infection.Pale gray ligands and receptors are required for optimal protective immunity to mycobacterial infection.Unfilled ligands and receptors were not required for normal granuloma formation and expression of protective immunity to mycobacterial infection.Unreported:Ligands and receptors whose function in granuloma formation and resistance to mycobacterial infection has not yet been reported.Life and death in the granuloma BM Saunders and WJ Britton106Immunology and Cell Biologyincluding MCP-1,MIP-1a,RANTES,IL-8and IP-10in the serum and bronchial alveolar lavage of TB patients,compared to healthy con-trols.62–66Human monocytes,macrophages and DCs infected with M.tuberculosis produce elevated levels of multiple chemokines, including MIP-1a,MIP-1b,MCP-1,MCP-3,RANTES,IP-10and IL-8.67–71There is also growing evidence that the expression of chemokines and the chemokine receptors CCR5and CXCR4are perturbed in HIV co-infected individuals,with increased and decreased expression depending upon the study and chemokine or receptor examined.This altered expression may be one mechanism leading to enhanced disease progression in HIV M.tuberculosis-co-infected individuals.72,73The exact requirement for individual chemokines and their recep-tors in generating protective granuloma formation during TB infec-tion remains uncertain.Studies utilizing gene-deficient mice have generally indicated a high degree of redundancy in this system (Table3).Indeed,the absence of individual chemokines is adequately compensated during murine M.tuberculosis infection.As most che-mokine receptors are bound by multiple ligands,these may be more critical to coordinate cellular inflammation and granuloma formation. For instance,CCR1binds several ligands,including MIP-1a,MIP-1b RANTES and MCP-3,whose expression is upregulated during myco-bacterial infection.74Nevertheless,CCR1À/Àmice control M.tubercu-losis infection normally,with no abnormality in granuloma size and structure(Saunders,unpublished observations).Indeed,of the che-mokine and chemokine receptor knockout mice challenged with M.tuberculosis(Table3),75–79only CCR2was essential to control high dose M.tuberculosis infection,76,78with aberrant granuloma formation a distinguishing feature of this enhanced susceptibility.Despite this redundancy in the effect of chemokines in mice, individual chemokines may be important in the resistance of humans to TB.For example,genetic analysis revealed that a single-nucleotide polymorphism in the MCP-1gene was associated with increased risk of developing pulmonary TB.80WT individuals and homozygotes for this mutation show an inverse relationship between MCP-1and IL-12p40levels.Interestingly,MCP-1À/Àmice show equivalent control of M.tuberculosis infection to WT mice,81suggesting that there may be differences in the requirements for individual chemokines and their receptors between humans and mice.WHEN GRANULOMAS FAILGranulomas fail to form in the absence of either adaptive IFN-g-secreting CD4T-cell responses or the co-locating signals provided by TNF and its family members.This is evident in gene-deficient strains of mice and subjects lacking functional IFN-g receptor/STAT1signal-ling.14But the majority of humans infected with M.tuberculosis control the initial infection through the formation of granulomas, which provide the microenvironment where specific T cells activate macrophages to contain the infection.Despite optimal activation, human macrophages fail to eradicate M.tuberculosis infection,and the dormant bacilli may persist for decades.In a minority of infected subjects,the granuloma fails for a variety of reasons,leading toTable2Chemokine expression in tissues from M.tuberculosis-infected individualsIn vivo/exvivo sample Chemokines Response ReferencePleura MIP-1a,MIP-1bMig,RANTESIP-10,MCP-1Increased expression in pleurafluid from TB patientsMIP-1a,Mig,RANTES and IP-10were expressed on pleural mononuclear cells and stroma cells,whereas CXCR3and CCR5were expressed on T cells in pleura64,115–117MCP-1,MIP-1a,MIP-1b Compared to TB infection alone,MCP-1expression increased in HIV-TB co-infected individuals,whereas MIP-1a,MIP-1b and RANTES expression was decreasedBALF IP-10,IL-8MCP-1,MCP-3,MCP-4RANTESIncreased in TB patients62,65,66,118,119MIP-1aExotaxinNot increased over healthy controlsLung MCP-1,MCP-3,MCP-4,IP-10Increased in TB patientsTGF-b,IFN-g and IL-12were also increased in TB granulomas,whereas IL-4and IL-10weredecreased66,120Eotaxin Not increasedAlveolar macropha-ges CCR5RANTESMIP-1aMCP-1Upregulated following M.tuberculosis infection in PBMC and alveolar macrophages65,74,121Plasma IL-8,IP-10Elevated in TB patients and in those with concurrent HIV infection73,117,122–124 MCP-1,RANTESMIP-1a,MIP-1bRANTESIP-10and MCP-1even higher in HIV-co-infected patients than TB infected individuals aloneMCP-1Not increased in all TB patients,higher in patients with extrapulmonary TBPBMC MIP-1aRANTES Increased in M.tuberculosis-infected PBMC from TB patients.Levels were lower in HIV co-infectedindividuals65,125126127MCP-1Increased mRNA and protein in monocytes from TB patients IL-8Increased following M.tuberculosis infectionCerebral spinalfluid MCP-1,IL-8,MIP-1a Elevated in CSF from patients with pyogenic and tuberculous meningitis128Life and death in the granulomaBM Saunders and WJ Britton107Immunology and Cell Biology。